Mortality from prolonged critical illness, including recovery from burn injury, remains high at least in part because of unchecked muscle catabolism that is difficult to prevent by nutrient supplementation alone. Despite adequate treatment of the underlying injury, this metabolic dysregulation can persist for months and contribute significantly to fatal complications. Intensive insulin therapy to restore euglycemia can reduce mortality and morbidity, but long-term insulin therapy can be difficult to implement safely. Thus, the elaboration of mechanistic-based strategies to modulate burn injury induced insulin resistance in skeletal muscle and liver is clinically important. The molecular basis of burn-induced insulin resistance is complex and involves dysregulation of both intracellular signaling and inter-tissue metabolism. Recently, we showed that mice lacking skeletal muscle Irsi and Irs2 display severe muscle catabolism. Whether the dysregulation of Irsi and Irs2 predisposes muscle to burn induced insulin resistance will be established in this project. In SA1, we will investigate the function of Irsi and Irs2 in skeletal muscle after burn injury using mice lacking, alternatively, Irs2 (MK02-mice) or Irsi (MKOI-mice). The potential for derepressed FoxOI or 3 transcriptional activity to cause muscle catabolism after burn injury in these genetically modified mice will be established via muscle specific knockout of FoxOI or 3 in MK02- or MKOI-mice. In SA2, we will investigate the functional significance of Irsi S/T-phosphorylation profiles in skeletal muscle following burn injury. Using our unique library of antibodies directed against specific phosphoserine sites of lrs1/2 together with Luminex detection technology, we will identify changes in the global and dynamic S/T-phosphorylation of Irsi that occurs after burns, including their relation to the metabolically important binding of PI-3Kto Irsi. The results obtained in mouse models will be compared directly with human skeletal muscle samples. In SA3, we will establish whether the phosphorylation of S307(lrs1) and S302(lrs1) regulates insulin signaling cascades in mice during burn injury. These experiments will be conducted using A307(lrs1) or A302(lrs1) knockin mice intercrossed with M0K2-mice, so that the results are uncomplicated by Irs2 signaling.